1. Field of the Invention
The invention relates to ac electric machines and more specifically to a stator an ac electric machine.
2. The Prior Art
Widely known in the prior art at present is a stator or an ac electric machine (EP 0271604), having a cylindrical magnetic circuit which occupies a set phase zone. The slots of the machine internally accommodate coil groups of two three-phase windings which are star and delta-connected and provided with three leads designed for connection to an external mains.
Previously, a star-connected three-phase winding is a main winding, while a delta-connected three-phase winding is an auxiliary winding with a substantially-reduced power (of up to two orders of magnitude). Both windings are connected in parallel to a supply source and are accommodated in common slots of the magnetic circuit. The coil groups of like phases of the three-phase windings are displaced relative to one another in the cross section of the stator through 90 electrical degrees and their corresponding leads are connected to different bus bars of a three-phase mains, where the time-phase displacement comprises 120.degree..
The embodiment and disposition of the three-phase windings described hereinbefore is intended to compensate for odd harmonics of an electromagnetic field of the electric machine stator. From this standpoint the characteristics of the stator electromagnetic field are upgraded, however this known design solution has no effect on the conversion of electrical energy into mechanical energy (operation in motor mode) and vice versa (operation in generator mode). In addition, there is no effect on the function of an electric machine as determined by speed-torque characteristics, torque ratio, current ratio, etc.
It is know that the electromagnetic force F in an ac electric machine applied to bus bars of the rotor depends on a magnitude of the vector of current I passing through the rotor bus bar and induced according to the electromagnetic induction law. The vector is induced by the magnetic flux of one phase of the stator on a magnitude of the vector of the magnetic induction B of the magnetic field of another phase of the stator, as well as on a geometric angle between the vectors which follows from Ampere's formula: EQU F=B.times.I.times.1 sin ,
where 1 is the active length of the rotor bus bar.
Thus, the optimum conditions for conversion of electrical energy into mechanical energy and vice versa will take place with the availability of geometric orthogonality and phase coincidence of vectors of the rotor current and stator magnetic induction.
At the same time, it is known that according to the electromagnetic induction law the rotor current lags from the magnetic field which induced the latter through a phase angle .pi./2. From this it follows that the optimum conversion of energy as applied to the stator of an electric machine described hereinbefore will take place in case when the geometric and phase angles between the vectors of magnetic induction of the adjacent phases of the stator will be equal to .pi./2. In other words geometric and phase orthogonality will provide optimum energy conversion.
In a known stator, the geometric and phase angles between said vectors of the magnetic induction comprise 90.degree. and 120.degree., respectively. Due to such relative orientation of the magnetic induction vectors the latter, in the general case, do not intersect the axis of a magnetic circuit. This leads to the fact that the optimum conditions for conversion of electrical energy into mechanical energy and vice versa are provided only in a narrow speed range (close to a synchronous speed or when an electric machine operates with a small slip). In addition, the use of said stator in an electric machine, particularly in an electric motor, prevents parametric control of the motor without changing the frequency of supply current of voltage.
As a rule, electric motors are designed for a rated mode of operation in which they have a high efficiency from 80% to 90%. Practically, such a mode is comparatively rare in practice and in the case of frequent starts of the motor, variations of voltage in the supply mains, and periodic operation at no-load. The actual efficiency of the electric motor turns out to be low, around 6-17.degree.. It should be noted that the electric motor with a known stator has a statistically unsteady speed-torque characteristic throughout 80% of the speed range and as a common induction motor, it has a torque ratio close to a unit and a current ratio of 6 to 7.5.
The known stator is characterized by a low manufacturability as the same slots of the magnetic circuit serve for accommodating the coils of both windings which ave conductors of different diameters. Such a process is labor intensive and makes it practically impossible to automate the assembly process.